This invention relates to the radio frequency (RF) processing of particulate dielectric materials, and more particularly to novel method and apparatus for the RF processing of such materials while simultaneously conveying the material between an inlet and an outlet.
The heating or other processing of a wide variety of particulate dielectric materials, such as thermoplastic and thermosetting synthetic resins, rubber and other elastomeric materials, and chemicals and food products, for such purposes as heating, dehydrating, devolutilizing, plasticating, sterilizing and pasteurizing, heretofore has been accomplished by a wide variety of means and methods. For example, heat has been provided by electrical resistance heaters, heat transfer liquids, radiation heating such as infrared, hot air, feed screw friction and other forms of conductive and shear heating. Such forms of heating are only about 20 to 30% efficient, reflecting correspondingly long residence time and thus correspondingly high energy costs. Adequate temperature control is difficult to obtain economically, and results in non-uniform heating of the materials, often causing thermal or oxidation degradation of properties.
The processing of such materials by radio frequency energy has been shown to be faster and less costly than the other heating methods discussed above. However, RF processing of particulate materials while simultaneously moving the materials between an inlet and an outlet, heretofore has been generally limited to the conveyor belt and vibratory type, or plunger type conveying systems, such as equipment for the injection molding of synthetic plastic resins. Typical of this type of equipment are those disclosed in U.S. Pat. Nos. 2,386,966 and 2,443,594.
Further, in all prior forms of RF processing of particulate materials while simultaneously conveying them between an inlet and an outlet, the precision and speed with which the temperature of the materials being processed is controlled, are deficient, resulting in high scrap rates and poor quality.
Most RF equipments of the prior art have incorporated various types of devices to control process temperature, with only minimal success. The reason for these deficiences is that most devices, such as variable vacuum capacitors, SCR controllers and electromechanical systems are too slow in responding to control signals, resulting in process temperature fluctations of 10.degree. F. to 20.degree. F. or more.
The most significant process variable which the aforementioned devices cannot efficiently respond to is power line voltage fluctuations, which are common across the country. In many instances line voltage changes by as much as .+-.25 volts or more, which in turn causes the process temperature to change by as much as .+-.50.degree. F.